1. Field of the Invention
The present invention relates to a belt for a continuously variable transmission, including a metal ring assembly and a plurality of metal elements formed from carbon steel and supported on the metal ring assembly. More particularly, the present invention relates to a belt having a carbon concentration in a recess of the metal element that is less than in other portions of the metal element.
2. Description of the Related Art
A metal element produced by subjecting a carbon steel, such as, for example, SKS95 or the like, to hardening and tempering treatment has been conventionally used as a metal element in forming a belt for a continuously variable transmission. The surface hardness of the conventional metal element is obtained by maintaining the carbon steel at a temperature equal to or greater than an austenitizing temperature, which is approximately 750° C., during the hardening treatment for at least 15 minutes.
The conventional metal element includes an element body that abuts a V-face of a pulley, a neck extending radially outward from a laterally central portion of the element body, and an ear leading to a radially outer side of the neck. The metal ring assembly is fit into a ring slot defined by the element body, the neck, and the ear. Because stress concentrates at the bottom of the ring slot, in particular, at a first connection between the element body and the neck, as well as a second connection between the ear and the neck, undercut portions are formed at the stress concentration regions of the ring slot to alleviate the concentration of stress therein.
Of the above-described two connections, the stress concentration typically occurs at the radially inner side, specifically, the first connection between the element body and the neck. The first connection is subjected to the harshest conditions in terms of strength. Japanese Utility Model Publication No. 5-14028 discloses a belt in which strength is enhanced by rounding the edge of the undercut portion formed at the first connection.
The surface of the metal element produced by subjecting the carbon steel to the hardening and tempering treatment obtains a high degree of hardness, which provides an enhanced durability against wear. Unfortunately, the enhanced hardness reduces the toughness of the metal element, which is then likely to suffer breakage or the like due to the concentration of stress. Therefore, considering that the toughness of carbon steel can be enhanced by reducing the carburization of the surface thereof, it is conceivable to enhance the fatigue strength and impact strength of the metal element by increasing the toughness of the portions of the metal at which stress typically concentrates.